1. Technical Field
One or more embodiments of the present invention relate to a surface light source device and, more particularly to a structure of a surface light source device for causing light to enter a light guide plate body thinner than the thickness of a light source efficiently.
2. Background Art
A surface light source device is required to have uniform luminance, to have high luminance, and to be low in cost, as well as to have a large light-emitting area (have a small non-light-emitting area) and to be thin. In particular, in a case where the surface light source is incorporated in a portable device, demand for thinning of the surface light source device is increasingly high according to thinning of the portable device.
In an edge-light type surface light source device, however, a point light source is disposed opposite to an end face of the light guide plate, and therefore, if the thickness of the light guide plate is made thinner than the height of a light exit window of the point light source, light not entering the light guide plate of light emitted from the point light source increases, and light-use efficiency of the surface light source device is reduced accordingly. Thus, the thickness of the light guide plate is restricted by the height of the light exit window of the point light source, and it is difficult to make the thickness of the light guide plate thinner than the height of the light exit window of the point light source accordingly. Similarly, in a case where the light source is a cold-cathode-fluorescent tube, it is difficult to make the thickness of the light guide plate thinner than the diameter of the cold-cathode tube.
(Regarding Patent Document 1)
In a liquid crystal display device disclosed in Japanese Unexamined Patent Application Publication No. 1993-53111 (Patent Document 1), in order to cause light from a fluorescent tube to efficiently enter a light guide plate thinner than the fluorescent tube, a tapered portion is provided at a thin portion of the light guide plate, namely, an end portion of a light guide plate body. An end face of the tapered portion has a height approximately equal to the diameter of the fluorescent tube, and the fluorescent tube faces the end face.
In such a liquid crystal display device, the thickness of the light guide plate body can be made thinner than the diameter of the fluorescent tube, so that light entering through the end face of the tapered portion is totally reflected by front and back faces of the tapered portion to be guided into the light guide plate body, and emitted from an upper face of the light guide plate body toward a liquid crystal panel.
In such a liquid crystal display device as described in the patent document 1, however, light leakage at the tapered portion cannot be prevented, and accordingly the light leaking from the tapered portion appears bright on an observer's side, and therefore an edge of a display portion (screen) of the liquid crystal display device emits light with high luminance to deteriorate the quality of the display portion. Even if the light leakage could have been prevented, in that case, the thickness of the light guide plate body cannot be made much thin as compared with the thickness of the tapered portion, or the length of the tapered portion must be made very long, which results in reduction of a light-emitting area.
(Regarding Patent Documents 2 and 3)
Japanese Unexamined Patent Application Publication No. 2004-69751 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. 2005-285389 (Patent Document 3) also disclose surface light source devices. In the surface light source device described in the patent document 2, however, light leaking out of the light guide plate cannot sufficiently be suppressed, and accordingly the leaked light shines on a display face of the liquid crystal display device to deteriorate the quality of the liquid crystal display device. Further, in the surface light source device described in the patent document 3, light is absorbed by a light reflecting plate, or light reflected by the light reflecting plate leaks from a light entering end face, which results in deterioration of light-use efficiency.
Incidentally, issues dealing with the surface light source devices of the patent documents 1, 2, and 3 are described in detail in a patent document 4.
(Regarding Patent Document 4)
Therefore, in the PCT application previously filed (PCT/JP2008/60610: Patent Document 4), the applicant of the present invention discloses a surface light source device that can make the thickness of the light guide plate body sufficiently thinner than the height of the light entering end face, and can further reduce light leakage from a tapered light introducing portion.
As shown in FIG. 1, the surface light source device 31 includes a point light source 32 using an LED, and a light guide plate 33. The light guide plate 33 includes a light introducing portion 35 provided at an end portion of a light guide plate body 34, and formed from transparent resin with a high refractive index. The light introducing portion 35 is thicker than the light guide plate body 34, and the point light source 32 is disposed so as to face an end face of the light introducing portion 35. On a face of the light guide plate body 34 positioned on the same side as a light exit face 39, a projecting portion having the shape of approximately a half of a circular truncated cone projects to increase the thickness of the light introducing portion 35. A peripheral face of the projecting portion forms an inclined face 37, and a directivity converting pattern 38 is formed on the inclined face 37. The directivity converting pattern 38 is obtained by arranging fine V-shaped groove structures 41 along the inclined face 37. When viewed from a direction perpendicular to the light exit face 39 of the light guide plate 33, the directivity converting pattern 38 constitutes a region having the shape of an arc around the center of a light exit window of the point light source 32 (near the center of the light source), and extended lines (longitudinal axes) in extending directions of the respective V-shaped groove structures 41 converge near the center of the light source. Note that on a face (back face) opposite to the light exit face 39 of the light guide plate body 34, many prismatic light emitting means 40 (see FIG. 2) are concentrically formed to reflect light guided in the light guide plate body 34 and emit the same from the light exit face 39.
Thus, as shown in FIG. 2, in the surface light source device 31, light L emitted from the point light source 32 enters the light introducing portion 35 from the light entering end face 36, is totally reflected by the directivity converting pattern 38 or a bottom face of the light introducing portion 35, or passes through the light introducing portion 35, and is guided to the thin light guide plate body 34. The light guided to the light guide plate body 34 is totally reflected or diffused by the light emitting means 40, and emitted approximately uniformly from the light exit face 39.
In the surface light source device 31 thus structured, light leakage from the light guide plate 33 can be eliminated in a plane perpendicular to the light exit face 39, for example, when setting is made in the following manner.
Refractive index of the light guide plate 33 n=1.59
Vertex angle of a ridge portion of the directivity converting pattern 38 φ=120°
Thickness of the end face of the light introducing portion 35 T=0.31 mm
Thickness of the light guide plate body 34 t=0.18 mm
Length of an upper face of the light introducing portion 35 s1=2.50 mm
Length of the light introducing portion 35 s2=3.19 mm
Inclination angle of the inclined face 37 θ=15.3°
Further, in a plan view of the light introducing portion 35 shown in FIG. 3A, an angle (hereinafter referred to as one-sided angle of view) ∠APC between a line segment AP connecting one end A of a light exit window 32a of the point light source 32 and the center P on the edge of the inner periphery of the directivity converting pattern 38 and a line segment CP connecting the center C of the light exit window 32a and the center P on the edge of the inner periphery of the directivity converting pattern 38 is set at 20°. Further, an angle (hereinafter referred to as one-sided angle of view) ∠BPC between a line segment BP connecting the other end B of the light exit window 32a of the point light source 32 and the center P on the edge of the inner periphery of the directivity converting pattern 38 and a line segment CP connecting the center C of the light exit window 32a and the center P on the edge of the inner periphery of the directivity converting pattern 38 is also set at 20°. In this example, an area occupied by the light introducing portion 35 increases, and a dead space S in the light guide plate 33 increases accordingly. However, lateral expansion of light incident on the center P of the directivity converting pattern 38 becomes 20° on each side of the centerline CP, lateral expansion of light outside the center P of the directivity converting pattern 38 is smaller than that, and therefore light leakage can be prevented by the whole of the directivity converting patter 38, so that leakage light in a plane parallel to the light exit face 39 is significantly reduced, and the percentage of maximum leakage light can be limited to 2% (that is, (leakage light)/(input light)≦2%).
Therefore, in the surface light source device 31 disclosed in the patent document 4, even if the inclination angle θ of the inclined face 37 is as large as 15.3°, light leakage from the light guide plate 33 can be significantly reduced by setting the one-sided angle of view of the light introducing portion 35 at 20°. Further, even if the one-sided angle of view is larger than 20°, it is possible to allow less leakage light by making the one-sided angle of view as smaller as possible.
However, in the market of surface light source devices, as well as the demand for thinning, demand for a light guide plate with a smaller dead space is increasing. Hence, for actual commercialization of product of the surface light source device, consideration is required to be made to reduce an area occupied by the light introducing portion.
The area occupied by the light introducing portion 35 can be reduced by designing the directivity converting pattern 38 to have a larger one-sided angle of view. For example, in the light introducing portion 35 shown in FIG. 3B, the one-sided angle of view ∠APC between the line segment AP connecting the one end A of the light exit window 32a of the point light source 32 and the center P on the edge of the inner periphery of the directivity converting pattern 38 and the line segment CP connecting the center C of the light exit window 32a and the center P on the edge of the inner periphery of the directivity converting pattern 38 is set at 30°. Similarly, the one-sided angle of view ∠BPC is also set at 30°. When the case shown in FIG. 3B where the one-sided angles of view ∠APC and ∠BPC are 30° and the case shown in FIG. 3A where they are 20° are compared with each other, in the case where the one-sided angles of view are 30° a dead space S for the light introducing portion 35 can be made smaller, but on the other hand the leakage light increases to a percentage of the maximum leakage light of 15% ((leakage light)/(input light)≦15%).
Furthermore, when the one-sided angle of view is 40°, the dead space can be smaller than that in the case of 30°, but the leakage light increases more than that in the case of 30°, and accordingly the percentage of the maximum leakage light becomes even higher than 15%.
An actual surface light source device is desired to have a smaller dead space even if the leakage light is sacrificed to some extent, but when the leakage light reaches about 15%, the light-use efficiency of the surface light source device decreases to cause reduction in luminance of a light emitting face (light exit face 39), and, as shown in FIG. 4, the light leaking from the directivity converting pattern 38 creates a light-emitting region J having high luminance at the edge of the light introducing portion 35, resulting in deterioration in quality of the surface light source device 31. Therefore, the leakage light should practically be limited to about 15% at the most, and accordingly, in the surface light source device 31 of the patent document 4, the dead space S cannot be reduced by making the area occupied by the light introducing portion 35 smaller than that in the case where the one-sided angle of view is 30°.